Sensing assemblies for sensing a process variable, whereas in the case of a temperature is to be sensed, they are given by thermometers or other related devices, are widely applied in the field automation and process techniques. Industrial environment thereby usually requires the sensing assemblies to have properties such as robustness, reliability, and high performance, in particular in case of hazardous process environments including e.g. high pressures and/or temperatures.
A thermometer is usually comprised by a measuring insert with a temperature probe located at its tip protected by a sheath. Additionally, the measuring insert may be inserted in a sleeve member, such as a thermowell, which is in particular done in the case of hazardous process environments. Such sleeve member may also be called protective tube. The temperature sensor itself may e.g. be a resistance thermometer as standardized according to DIN 43735. However, many different temperature sensors and according DIN versions as well as customer specific ones are available on the market and are known in the state of the art.
For example, patent application U.S. Pat. No. 2,930,827 describes a thermometer with thermocouple wires packed in magnesium oxide insulators. The insulators are spaced from each other, form chambers and are arranged in a tubular shield. Other known arrangements comprise inserting one or more temperature sensors into a sheath and embedding it/them in a refractory material which is poured into the sheath as e. g. disclosed in patents GB 2183909 and JP S57103025.
If the process variable, e. g. temperature, is to be sensed at a specific location, i. e. a specific measuring point, inside a large volume (vessel, tank or other) the respective sensor element, e. g. a temperature probe, has to be guided towards that location, usually along a predefined path, e. g. parallel to walls or parallel to other internal structures within the volume. In the case, that more than one measuring points are of interest, a multipoint sensing assembly is needed, comprising a multitude of temperature probes guided to each of the measuring points of interest.
A multipoint measuring assembly for sensing temperature is e. g. described within patent application U.S. Pat. No. 6,550,963 B2. A multipoint thermocouple comprises a sheath having a plurality of conductor pairs disposed within a sheath. This sensing assembly does not feature any sleeve member, so that the sheath of the measuring insert represents the only barrier towards the process environment. For the ability to withstand the process conditions, the sheath is usually chosen to have a rather high thickness in such arrangements. As a consequence, the temperature probe becomes rather rigid making its installation difficult due to frequent necessity of bending it along its length in order to reach the respective measuring point. Furthermore, because the sheath is an integral part of the temperature probe, it might be easily damaged while in direct contact with the process medium. Finally, a replacement in case of failure is not possible.
To overcome such disadvantages, at least one sleeve member may be implemented into the set-up. This leads to a double barrier between the temperature probe and process medium in the form of the sleeve member and the sheath of the measuring insert and thus provides an improved protection of the temperature probe, especially with respect to hazardous media. Because lacking the necessity of a large diameter to withstand heavy process conditions, the measuring insert and/or temperature probe might be constructed flexible making its handling easier.
However, in this concern, the geometry of the at least one sleeve member, as well as the necessary amount of sleeve members to reach all measuring points of interest, depends on their relative distribution, in particular on their spatial distribution, within the vessel or tank. Depending on the actual situation, one of at least two cases may occur: First, all measuring points of interest may be connectable by a more or less straight line, e. g. parallel to the wall of the vessel or tank so that only one sleeve member is needed. In this case, it is possible to construct and manufacture the entire multipoint measuring assembly in a supplier factory before introducing it via an insertion point into the vessel or tank. Alternatively, also the sleeve member may be first introduced and fastened within the vessel or tank and subsequently inserting the measuring insert. In the second much more frequent case, in turn, the measuring points of interest may not be connectable by one more or less straight line but may develop along one or more three dimensional paths proceeding through the interior of the tank or vessel. Then, the installation of the at least one sleeve member needed becomes much more difficult and complex. In particular, the sleeve members usually need to be inserted into the tank or vessel already during its design and/or construction phase. Thereby, the at least one sleeve member hast to be bended and eventually even needs to be constructed by more than one part, whereas the different parts are e. g. welded together to form one sleeve member.
Even though a multipoint measuring assembly comprising at least one sleeve member may enable for a straightforward exchange of the at least one measuring insert including the various temperature probes in case of damage, the sleeve member in the majority of cases needs to be implemented and assembled within the vessel or tank during the design and/or construction phase already. As a consequence, functional testing has to be performed on site after complete installation and eventually subsequent changes are very difficult to carry out.
It is hence an object of the invention to provide a sensing assembly and a multipoint sensing assembly, preferably for sensing temperature, which allows for an easy on-site installation.